Recently, to lighten the weight of airplanes, automobiles, etc., opportunities to employ fiber reinforced plastic used with carbon fibers (hereinafter, also referred to as “CFRP”) have increased. For example, a CFRP using a carbon fiber woven fabric as a reinforcing fiber substrate, in which carbon fiber bundles are used, each comprising a plurality of carbon fibers arranged in one direction, is very advantageous in specific stiffness and specific strength as compared to metal materials, and it is employed for various parts.
As a process for molding such a CFRP, various processes are proposed such as prepreg/auto clave process, RTM (Resin Transfer Molding) process, RFI (Resin Film Infusion) process, processes from derived therefrom, etc. Among these processes, RTM process is paid attention to from a viewpoint of being able to obtain a CFRP having a complicated shape, for example, by preparing a carbon fiber substrate formed from a fabric material comprising carbon fibers or a laminate stacked with a plurality of the fabric materials, forming a preform with a predetermined shape in advance, and impregnating a matrix resin injected into a mold into the preform and curing the resin.
However, in a case where there is an attempt to cut the fabric or the laminate thereof comprising carbon fibers, or the preform formed therewith at a predetermined shape in advance (hereinafter, these are also called as a “carbon fiber substrate” as a generic term), into a predetermined shape by using a usual cutter, the following problems may occur. Namely, because a carbon fiber is very thin such as about 10 μm in diameter, in a case where it is attempted to cut the laminate of carbon fiber fabrics or the preform thereof by a cutter at a contact condition, the portion to be cut is cut under crushing conditions, also because of the hardness of carbon fiber itself. Therefore, by the repelling power, the carbon fibers are liable to be frayed at a cutting end surface. In particular, when the cutting is attempted after laminating fabrics and forming the preform, the cutting surface is liable to become uneven in the thickness direction.
If such a preform is disposed into a mold, a mismatch may occur between it and the cavity shape of the mold. In a case where the preform is larger than the mold, an additional cutting processing operation is conducted to adjust the shape to a size fitting to the mold, or to perform molding by containing the preform larger than the mold in the mold as it is. In the latter case, carbon fibers are contained up to a flash portion of CFRP after molding, and there occurs an inconvenience that the flash removing operation becomes troublesome. On the other hand, in a case where the preform is smaller, because a portion of only resin (resin rich portion) is formed in a gap between the preform and the mold, an operation for charging carbon fibers separately becomes necessary before injection of matrix resin. Further, even if the shapes of the preform and the mold almost coincide with each other, an end portion of the preform may be frayed, for example, when the preform is transferred to the mold and, therefore, it is difficult to completely suppress the mismatch.
For such carbon fibers that are easily frayed, a technology is known wherein carbon fibers are to be bound with one another. For example, as a technology for binding carbon fibers, a process for obtaining a sheet-like molded material from carbon fibers formed as short fibers (about 3-20 mm) via a phenolic resin and the like is described in JP-A-2004-288489 and JP-A-2005-297547. As a method for making this sheet-like molded material, a method is described wherein short carbon fibers are dispersed randomly in a two-dimensional plane, and they are calcined together with the phenolic resin in an inert atmosphere at a high temperature of about 200° C. or higher. The sheet-like molded material described in those publications is used suitably for a carbon fiber electrode, and it is not used by additionally impregnating a matrix resin into the sheet-like molded material. Further, since the whole of the short carbon fibers themselves are carbonized because of the calcination at a high temperature of about 200° C. or higher, the elastic modulus, the strength, etc. of the short carbon fibers themselves cannot be exhibited.
To address the problem as aforementioned that the cutting end surface is liable to be frayed and the carbon fiber substrate is hard to be precisely cut at a predetermined shape when the substrate is cut using a usual cutter, although not published, a technology has been proposed wherein by generating a specified-nature graphitized portion (for example, membrane-like graphitized portion) on the cutting end surface, while achieving an easy cutting to a predetermined shape, fraying of carbon fibers at the cutting end surface and the like can be prevented (Japanese Patent Application No. 2000-285882), and in that proposal, it is also described that such a cutting may be achieved by cutting with a laser (laser ray).
However, it has become clear that there remain the following problems in case the carbon fiber substrate is thus cut by a laser. First, in a laser processing, because the sublimation cutting temperature of carbon fibers is about 3,800° C. to be higher than that of a metal (about 1600° C. in the case of iron) and a difference between an atmosphere temperature and the cutting temperature is great, the thermal energy is liable to be dissipated. Although the thermal conductivity of carbon fiber itself is higher than that of a general inorganic substance, depending upon the formation of a substrate formed from a fabric material, because there is a case where the thermal conductivity of the substrate in a cutting processing direction, that is, a thickness direction (especially, a thickness direction of a substrate comprising a laminate) becomes extremely low as compared with that of a metal, heat supply through the substrate itself in the thickness direction is hardly achieved, and it may become difficult to reach the cutting temperature of the substrate. If the cutting temperature by a laser is not sufficiently elevated, namely, if it is not elevated up to the sublimation cutting temperature of carbon fibers, a cutting defective place may occur.
Further, in the laser cutting, there is a focal distance capable of processing (a focal distance for convergence), and in a case where the substrate is one easy to be deformed, there is a fear that the portion to be cut may shift from the range of the focal distance capable of processing and it shifts, a cutting defective place also may occur.
Accordingly, to address the above-described problems when the carbon fiber substrate is cut by a laser and other problems accompanying with the laser cutting, it could be helpful to provide a method for cutting a carbon fiber substrate capable of performing a laser cutting, that can expect an excellent cutting performance and an excellent cutting end surface form as aforementioned for cutting of a carbon fiber substrate, stably at a target desired condition.